CN1277692C - Ink-jet recording medium and method for production thereof - Google Patents

Abstract

The object of the present invention is to provide an ink jet recording medium which is high in gloss and ink absorption and excellent in image colorfulness and adhesion of the coats and a method for producing the same. According to the present invention, there is provided an ink jet recording medium which comprises a support, an undercoat layer containing a salt of an alkaline earth metal and an adhesive provided on the support and an ink-receiving layer provided by coating a coating solution containing inorganic ultrafine particles on the undercoat layer. The undercoat layer preferably contains the adhesive in an amount of 0.05-0.8 time the amount of the salt of alkaline earth metal in weight ratio. Furthermore, the undercoat layer preferably contains an organic pigment. The alkaline earth metal is preferably calcium or magnesium, and more preferably the salt of the alkaline earth metal is a carbonate. The inorganic ultrafine particles are of an amorphous synthetic silica produced by a gas phase method or an alumina compound. There is further provided a method for producing the ink jet recording medium according to which the ink-receiving layer is provided after the undercoat layer is coated and subjected to a hot calendering treatment.

Description

Inkjet recording medium and manufacturing method thereof

technical field

The present invention relates to an inkjet recording medium and a method for producing the same, and more particularly to such an inkjet recording medium comprising a support, an undercoat layer on the support, and an inkjet receiving layer on the undercoat layer (which is formed by coating a coating solution containing inorganic ultrafine particles for the ink receiving layer). This inkjet recording medium has excellent coating adhesion, and has high gloss, high ink absorption, excellent image color and high smoothness.

Background technique

The inkjet recording system ejects ink droplets according to various operating principles and deposits them on a recording medium such as paper, thereby accomplishing the recording of images or letters. Recording systems have the characteristics that they perform these operations at high speed, generate little noise, can easily perform multi-color printing, have great versatility in recorded graphics, do not require development-fixation, and recently these The system rapidly spread as a recording device to various applications for recording various graphics and color images including Chinese characters. Furthermore, images obtained by a multicolor inkjet system are not inferior in quality to recorded images obtained by multicolor printing by a platemaking system or a color photographic system. Also, when producing small quantities of prints, they can produce cheaper prints than photographic techniques. Therefore, inkjet recording systems have been widely used in the field of full-color image recording.

Also, due to the variety of uses, they can be used to prepare large-scale posters and popular artworks and drafting drawings. In these uses, satisfactory images can be obtained due to the use of high definition and excellent color obtained from inkjet recording (for better publicity effect). These applications are increasing because images with excellent image reproducibility or color reproducibility such as sharpness and color are readily available on personal computers, which is why inkjet recording media are more used.

Due to the increased performance and versatility using inkjet recording devices, the characteristics required for inkjet recording media have also significantly increased. In particular, it is advertised that the recording device can produce high-definition images, and devices capable of producing large-scale prints require an extremely large amount of ink to form images compared to conventional devices, and research is currently underway to improve the ink-receiving layer for absorbing ink.

In addition, the diversity of uses affects the appearance of inkjet recording media. In addition to the appearance of no or low gloss (such as common plain paper or matte paper), there are needs such as fine art paper, coated paper, casting paper and photographic paper. Such a shiny look. This is because inkjet recording is required to reproduce image quality corresponding to prints and photographs, and an appearance similar to prints and photographs.

As an inkjet recording medium having a glossy surface, JP-A-6-320857 discloses a cast-coated paper obtained by performing cast-finishing while the coating is in a wet state . However, the surface gloss is much lower than that of silver halide paper, and the texture of silver halide paper cannot be obtained.

As inkjet recording media having enhanced surface gloss, media including a support and an ink-receiving layer including a resin on the support have been proposed. As examples of resins for this use, various ink-absorbing polymers have been proposed, such as polyvinylpyrrolidone and vinylpyrrolidone-vinyl acetate copolymers (such as JP-A-57-38185 and JP-A-62-184879 ), the resin composition mainly includes polyvinyl alcohol (as described in JP-A-60-168651, JP-A-60-171143 and JP-A-61-134290), vinyl alcohol, olefin or styrene and Copolymers of maleic anhydride (as described in JP-A-60-234879), polyethylene oxide crosslinked with isocyanate (as described in JP-A-61-74879), carboxymethylcellulose and polycyclic A mixture of ethylene oxide (as described in JP-A-61-181679), a polymer obtained by grafting methacrylamide on polyvinyl alcohol (as described in JP-A-61-132377), a carboxyl group-containing Acrylic polymers (as described in JP-A-62-220383), polyvinyl acetal (as described in JP-A-4-214382) and crosslinkable acrylic polymers (as described in JP-A-4-282282 and described in JP-A-4-285650). Furthermore, JP-A-4-282282 and JP-A-4-285650 propose an inkjet recording medium comprising a polymer matrix in which a crosslinkable polymer and an ink-absorbing polymer are mixed. However, although the ink-receiving layer includes these resins having a glossy surface, they have some disadvantages in that they have a low absorption rate and a small amount of absorption compared to ink-receiving layers containing pigment fine particles such as silica. .

As an inkjet recording medium having a high absorption rate and improved surface gloss, an inkjet recording medium using hydrated alumina (cationic hydrated alumina) has recently been proposed, for example, JP-A-60-232990, JP-A-60-232990, JP-A-60-232990, -A-60-245588, JP-B-3-24906, JP-A-6-199035 and JP-A-7-82694 propose an inkjet recording medium comprising a microfine false film coated thereon A carrier for hydrated alumina of the amite type and a water-soluble binder. However, although inkjet recording media using fine pseudo-boehmite-type hydrated alumina have very high surface gloss, they have a small pore volume, and therefore, as described in JP-A-5-24335, they have relatively high Small ink absorbency, and thick coats must be used to obtain adequate ink absorbency.

Also, JP-A-10-203006 and JP-A-8-174992 propose an inkjet recording medium using synthetic carbon dioxide having a primary particle diameter of 3 to 30 nm and mainly prepared by a gas phase method. silicon. In this case, too, a coating thickness of 30 microns or greater must be used to obtain adequate absorbency. Also, JP-A-11-48602 proposes an inkjet recording medium having a first ink-absorbing layer comprising a hydrophilic binder and solid particles of 0.5 to 2.5 times the weight of the hydrophilic binder and having A dry film with a thickness of 5-30 microns and a void layer containing particles with an average particle size not exceeding 100 nanometers. However, in this case, the interstitial layer must be thick enough to get enough ink absorption, and if the layer is made thicker, defects (such as cracks) will occur. In order to balance thickness and ink absorption, in the first The amount of binder in the ink-absorbing layer must be greater, but this first ink-absorbing layer does not improve ink absorption.

On the other hand, there is a proposal to obtain the sharpness of the image originally by considering the paper having an ink-receiving layer thereon. For example, JP-A-62-162588 proposes an inkjet recording medium having a sheet whiteness of not less than 90% comprising bleached pulp of not less than 90% whiteness and a filler material having absorbent properties. However, according to this method, sufficient whiteness and absorption can be obtained, but high-quality glossy images cannot be obtained. Also, JP-A-11-129613 proposes a recording paper having a layer containing solid particles having a fluorescent whitening effect. However, this method requires a step of providing a layer containing a specific component or a step of adding a specific component in the ink-receiving layer, thereby complicating the production process, and is not preferable.

Furthermore, JP-A-4-204727 and JP-A-4-296745 propose resin-coated supports for photography with improved whiteness. However, these techniques are only aimed at the production of photographic paper, and since the paper is coated with resin, the ink absorption capacity is small, and in order to obtain sufficient ink absorption capacity, the ink receiving layer must be thick. JP-A-2000-33771 proposes an ink recording medium comprising a layer of base paper having a layer comprising barium sulfate and thermoplastic hollow beads on one side, the base paper being coated with a layer comprising polyvinyl alcohol, polyethylene Ink-receiving layer of pyrrolidone-based and vinyl acetate-butyl acrylate copolymer. However, since the ink receiving layer contains a resin, it has a glossy surface, but it has a lower absorption rate and a smaller absorption amount than an ink receiving layer containing pigment particles such as silica.

Contents of the invention

The object of the present invention is to provide an ink jet recording medium which has high gloss and ink absorbency, excellent color and has no problem in the adhesiveness of the ink receiving layer, and also provides a method for preparing the ink jet recording medium In addition, an inkjet recording medium having excellent smoothness and a method for producing the same are provided.

The present invention provides an inkjet recording medium comprising a support, an undercoat layer on the support, and an ink receiving layer formed thereon by coating a coating solution containing inorganic ultrafine particles, wherein the undercoat The layer contains an alkaline earth metal salt and a binder.

In the primer layer, the preferred binder content of the binder is 0.05-0.8 times the weight of the alkaline earth metal salt.

The alkaline earth metal is preferably calcium or magnesium, and the alkaline earth metal salt is more preferably carbonate.

The inorganic ultrafine particles are preferably amorphous synthetic silica or alumina compounds prepared by a gas phase method.

The coating solution for the ink-receiving layer containing inorganic ultrafine particles preferably has a pH of not higher than 5.0.

A preferred example is that, in the primer layer, the content of the binder is 0.05-0.4 times the weight of the alkaline earth metal salt.

Furthermore, the present invention provides an inkjet recording medium comprising a support, an undercoat layer on the support, and an ink receiving layer formed by coating a coating solution containing inorganic ultrafine particles on the undercoat layer, wherein the undercoat The layers also contain organic pigments in addition to alkaline earth metal salts and binders.

The alkaline earth metal is preferably calcium or magnesium, and the alkaline earth metal salt is more preferably carbonate.

A preferred example is that, in the primer layer, the content of the organic pigment is 0.05-20 times the weight of the alkaline earth metal salt.

The organic pigments are preferably hollow organic pigments or dense organic pigments, which can also be mixtures of said pigments. In the case of using a mixture, a preferable example is that, in the mixture, the content of the dense organic pigment is 0.1 to 10 times the weight of the hollow organic pigment.

The organic pigment is preferably a hollow organic pigment having an average pore content of not less than 20%.

The organic pigment is preferably a bowl-shaped compact organic pigment.

The organic pigment preferably has an average particle diameter of 0.3-10 microns.

The inorganic ultrafine particles are preferably amorphous synthetic silica or alumina compounds prepared by a gas phase method.

The coating solution for the ink-receiving layer containing inorganic ultrafine particles preferably has a pH of not higher than 5.0.

A preferred example is that, in the primer layer, the content of the binder is 0.05-0.8 times the total weight of the alkaline earth metal salt and the organic pigment.

Furthermore, the present invention provides a method for producing an ink jet recording medium, which comprises forming an undercoat layer on a support and forming an ink receiving layer by coating a coating solution containing inorganic ultrafine particles on the undercoat layer, wherein The undercoat layer contains alkaline earth metal salt and a binder of 0.05-0.8 times the weight of the alkaline earth metal salt, and after forming the undercoat layer, the undercoat layer is subjected to heat calendering treatment, and then an ink receiving layer containing inorganic ultrafine particles is formed.

In addition, the present invention provides a method for producing an ink recording medium, which comprises forming an undercoat layer on a support and forming an ink receiving layer by coating a coating solution containing inorganic ultrafine particles on the undercoat layer, wherein the undercoat layer The coating contains alkaline earth metal salts, organic pigments and 0.05-0.8 times the total solid weight of alkaline earth metal salts and organic pigments. Particle ink receiving layer.

In the method for preparing the above-mentioned inkjet recording medium, it is preferable that the inorganic ultrafine particles are amorphous synthetic silica or alumina compounds prepared by a gas phase method, and that the pH of the coating solution for the ink receiving layer is not high at 5.0.

Detailed ways

The inkjet recording medium of the present invention and its preparation method will be described in detail below.

The inkjet recording medium of the present invention comprises a support, an undercoat layer on the support, and an ink-receiving layer on the undercoat layer comprising inorganic ultrafine particles, preferably silica or oxide prepared by a gas phase method. Aluminum compounds.

The undercoat layer of the present invention comprises an alkaline earth metal salt. The alkaline earth metal in the present invention is a general term for beryllium, calcium, magnesium, strontium, barium and radium. Alkaline earth metal salts include carbonates, silicates, borates, hydrochlorides, sulfates, organic acid salts, etc., and weak salts with low solubility are preferred, because in many cases, the primer used The coating solution is an aqueous solution. Especially preferred are carbonates, examples of which are calcium carbonate, magnesium carbonate and the like.

The particle shape of the alkaline earth metal salt is urchin-shaped, square, cylindrical, amorphous, spherical, etc., and any of them can be satisfactorily used.

In the primer layer containing alkaline earth metal salt, the content of binder is 0.05-0.8 times of the weight of alkaline earth metal salt. More preferably 0.05-0.4 times. If the amount of the binder is less than 0.05 times the weight of the alkaline earth metal salt, adhesion will be insufficient and separation will occur between the undercoat layer and the carrier or ink receiving layer. If the amount of the binder exceeds 0.8 times the weight of the alkaline earth metal salt, the absorption will be poor, which is not preferable.

When the undercoat layer of the present invention contains an organic pigment, examples of the organic pigment used include thermoplastic resins (such as polystyrene resins, styrene-acrylic resins, acrylic resins, polyethylene resins, vinyl acetate-based copolymer polyolefin resins, etc.) , polypropylene resin, polyacetal resin, chlorinated polyether resin and polyvinyl chloride resin. Organic pigments can have a multilayer structure of these resins. Among these resins, polystyrene resins, acrylic resins or styrene- Acrylic.

Among these organic pigments, pigments having an average particle diameter in the range of 0.3 to 10 µm are preferred. The average particle diameter is more preferably 0.3-6 microns. If the average particle diameter is less than 0.3 microns, the organic pigment will be tightly packed in the undercoat layer, impairing ink absorption, which is not preferable. If the average particle diameter exceeds 10 micrometers, since the particle diameter of the organic pigment is large, the number of pores in the undercoat layer decreases, thereby impairing ink absorbency, which is not preferable.

The shape of the organic pigment used in the present invention may be any of solid spherical shape (ie, without voids), hollow spherical shape, bowl shape, red blood cell shape, confeitos, etc., and two or more of these shapes may be used in combination. From the viewpoint of ink absorbability, preferred are hollow organic pigments having one or more void (hollow) portions in particles and bowl-shaped dense organic pigments obtained by cutting a part of almost perfectly spherical hollow organic pigments. The average void content of the hollow organic pigment is preferably not less than 20%. Void content refers to the ratio of the volume of voids to the volume of organic pigments. As these hollow organic pigments and bowl-shaped dense organic pigments, commercially available organic pigments can be suitably used. Examples of commercially available hollow organic pigments are ROPAQUE HP-1055, HP-91, OP-84J, and HP-433J (manufactured by Rohm & Haas Co., Ltd), and examples of these dense organic pigments are L880l (manufactured by Asahi Kasei Kogyo K.K.) and ARTPEARL F-4P (manufactured by Negami Kogyo Co., Ltd), examples of these bowl-shaped dense organic pigments are V2005 (manufactured by Nippon Zeon Co., Ltd) and the like. In the case of using a mixture of hollow organic pigments and dense organic pigments, the mixture preferably contains 0.1 to 10 times the weight of the hollow organic pigments as dense organic pigments.

In the case where the undercoat layer of the present invention contains an organic pigment, the undercoat layer contains the organic pigment in an amount of 0.05 to 20 times the weight of the alkaline earth metal salt. The amount of organic pigment is more preferably 0.05-6 times. If the amount of the organic pigment is less than 0.05 times the weight of the alkaline earth metal salt, gloss and smoothness will be deteriorated, which is not preferable. If the amount of the organic pigment exceeds 20 times the weight of the alkaline earth metal salt, the ink absorbency will be poor, which is not preferable.

In an example where the undercoat layer of the present invention includes an alkaline earth metal salt and an organic pigment, the undercoat layer includes a binder in an amount of 0.05-0.8 times the total solid weight of the alkaline earth metal salt and the organic pigment. A more preferable range is 0.05-0.4 times. If the amount of the binder is less than 0.05 times the weight, the adhesion will be insufficient and separation will occur between the undercoat layer and the carrier or ink-receiving layer. If the amount of the binder exceeds 0.8 times the total solid weight of the alkaline earth metal salt and the organic pigment, it will cause poor absorbency, which is not preferable.

Binders included in the base coat include cellulosic binders (such as methylcellulose, methylhydroxyethylcellulose, methylhydroxypropylcellulose, and hydroxyethylcellulose), natural polymer resins or its derivatives (such as starch and its modified products, gelatin and its modified products, casein, pullulan, gum arabic and albumin), polyvinyl alcohol and its modified products, latex or emulsion (such as styrene - butadiene copolymers, styrene-acrylate copolymers, methyl methacrylate-butadiene copolymers and ethylene-vinyl acetate copolymers), vinyl polymers such as polyacrylamide and polyvinylpyrrolidone ), polyethyleneimine, polypropylene glycol, polyethylene glycol, maleic anhydride or its copolymers, etc. Among them, copolymer emulsions are preferable.

In addition, acrylic resin adhesives (which are synthetic resins obtained by polymerizing acrylic acid, acrylate, acrylonitrile, etc.) and their emulsions can be used as adhesives, which give printed parts an excellent sense of gloss because they They have excellent light resistance and suppress discoloration, resulting in improved light resistance of the white paper portion of inkjet recording media, and they have high transparency. Specifically, emulsion-type acrylic resin adhesives have excellent adhesive strength.

And, as long as the realization of the object of the present invention is not hindered, other additives can be optionally added, such as cationic color fixing agent, pigment dispersant, thickener, flowability improver, viscosity stabilizer, pH regulator, surface active agent, disinfectant Foaming agent, antifoaming agent, antisticking agent, foaming agent, penetrating agent, colored dye, colored pigment, white inorganic pigment, white organic pigment, fluorescent whitening agent, ultraviolet absorber, antioxidant, leveling agent, anticorrosion agent, antifungal agent, water repellant, dryness enhancer and wetting enhancer.

The method of applying the undercoat layer is not particularly limited, and known coating methods can be used. For example, the base coat can be applied to the support using a variety of equipment including air knife coaters, curtain coaters, slide lip coaters, die coaters , scraper coater, gate roller coater, bar coater, rod coater, roll coater, billblade coater, short stay scraper coater machine (short dwell blade coater) and sizing printing press.

In addition, the applied base coat can be ironed out by calendering. In this case, the calendering treatment includes gloss calendering, super calendering, soft calendering, and the like. Especially preferred is the hot calendering treatment, which is ironed with heat.

Moreover, when the calendering treatment using a roller with a rough surface is performed as a calendering treatment of the undercoat layer to roughen the surface of the undercoat layer, the gloss of the white paper on the surface of the recording medium will decrease, and a sense of gloss will be given to the printed part. , while having a matte surface.

In this case, if the roughness of the rough surface roller is too high, the smoothness of the surface of the recording medium is significantly reduced, resulting in poor quality of printed images. Therefore, in the ten-point average roughness (R _z ) according to JIS-B-0601, the roughness of the rough-surfaced roller is preferably 1 to 40 micrometers, more preferably 1 to 30 micrometers.

There is no specific limit to the coating amount of the primer layer, but if it is too small, the effect of the primer layer cannot be expanded, and if it is too much, not only will there be difficulties in production, but the effect will be saturated, and the economic efficiency will be reduced. reduce. Therefore, the coating amount is preferably 5-30 g/ ^m2 .

In the inkjet recording medium of the present invention, the inorganic ultrafine particles have a primary particle diameter of 100 nm or less and a secondary particle diameter of 400 nm or less. As typical examples of inorganic ultrafine particles, there can be mentioned pseudoboehmite sol (which is hydrated alumina, such as JP-A-1-97678, JP-A-2-275510, JP-A-3-281383, JP-A-3-281383, -As described in A-3-285814, JP-A-3-285815, JP-A-4-92183, JP-A-267180 and JP-A-4-275917), colloidal silica (as described in JP-A- 60-219083, JP-A-61-19389, JP-A-61-188183, JP-A-63-178074 and JP-A-5-51470), silica/alumina hybrid (hybrid) Sols (such as JP-B-4-19037 and JP-A-62-286787), silica sols prepared by dispersing silica by a gas phase method using a high-speed homogenizer (such as JP-A-10-119423 and JP-A-10-217601) and other smectite clay (such as hectite and montmorillonite (JP-A-7-81210), zirconia sol, chromia (chromia) sol, yttrium oxide sol, Ceria sol, iron oxide sol, zircon sol and antimony oxide sol.

Among these inorganic ultrafine particles, particularly preferred are silica ultrafine particles and alumina compounds (hydrated alumina or alumina ultrafine particles) produced by a gas phase method.

Silica fine particles are fine particles comprising 93% or more of SiO ₂ , about 5% or less of Al ₂ O ₃ and about 5% or less of Na ₂ O by dry weight, here there are amorphous silica, Such as white carbon, silica gel and finely powdered silica. Amorphous silica fine particles are produced by a liquid phase method, a grinding solid phase method, a crystalline solid phase method and a gas phase method. The liquid phase method can produce fine particles by precipitating a solid silicic acid compound in a so-called liquid by a chemical change or a physical change. The grinding solid phase method is a method of mechanically grinding silica solids, and the crystallization solid phase method can use the melting or phase change of solids to prepare fine particles. Gas-phase methods can produce fine particles by thermal decomposition of volatile metal compound vapors or by heating and evaporating raw materials, cooling and condensing the resulting vapors.

The silica fine particles used in the present invention are amorphous silica fine particles synthesized by a gas phase method. Among them, ultrafine particle silica having an average primary particle diameter of 3 to 50 nm is preferred. Especially preferred is ultrafine particle silica having a primary particle diameter of 5-30 nm. The secondary particle diameter of the particles formed by linking them is preferably 10 to 400 nm. A commercial product of amorphous silica fine particles synthesized by the gas phase method is AEROSIL (manufactured by Degussa Co., Ltd.).

Silica obtained by using the gas-phase method in the present invention is produced by adding silica fine particles having the above-mentioned primary particle diameter to water, and dispersing them using a high-speed homogenizer or the like so that the average secondary particle diameter 400 nm or less, preferably 200 nm or less.

The hydrated alumina used in the present invention can be represented by the following formula:

Al ₂ O ₃ nH ₂ O

According to the different composition or crystalline form, hydrated alumina can be divided into gibbsite, magnesium phosphate mayenite, new trihydrate alumina (norstrandite), boehmite, boehmite gel (pseudoboehmite), water Aluminum stone, amorphous hydrate, etc. When n in the above formula is 1, the formula represents hydrated alumina with a boehmite structure; when n is greater than 1 and less than 3, the formula represents hydrated alumina with a pseudo-boehmite structure; when n is 3 or greater , which represents an amorphous structure of hydrated alumina. In the present invention, particularly preferred hydrated alumina is a pseudoboehmite structure having the above formula, wherein n is greater than 1 and less than 3.

The shape of the hydrated alumina used in the present invention may be any of flake, fibrous, needle, spherical and rod, and is preferably flake from the viewpoint of ink absorption. The average aspect ratio of the flaky alumina hydrate is 3-8, preferably 3-6. The aspect ratio is the ratio of the "diameter" to the "thickness" of a particle. The diameter of a particle is the diameter of a circle having an area equal to the projected area of the particle as viewed in an electron microscope. If the aspect ratio is smaller than the above range, the pore size distribution of the ink-receiving layer becomes narrow, and the ink absorption becomes poor. If it is larger than the above range, it is difficult to produce hydrated alumina of uniform particle size.

The hydrated alumina used in the present invention is prepared by known methods including hydrolysis of aluminum alkoxides such as aluminum isopropoxide, neutralization of aluminum salts with alkali, and hydrolysis of aluminates. The physical properties of hydrated alumina, such as particle size, pore size, pore volume, and specific surface area, are controlled by conditions including precipitation temperature, aging temperature, aging time, solution pH, solution concentration, and coexisting compounds.

As methods for producing hydrated alumina from alkoxides, JP-A-57-88074, JP-A-62-5632l, JP-A-4-275917, JP-A-6-64918, JP-A-7- 10535 and JP-A-7-267633 and USP265632l proposed the method of hydrolyzing aluminum alkoxide. Examples of aluminum alkoxides are isopropyl oxide and 2-butyl oxide.

Furthermore, JP-A-54-116398, JP-A-55-23034, JP-A-55-27824 and JP-A-56-120508 propose methods using an inorganic salt of aluminum or a hydrate thereof as a raw material. Raw materials include inorganic salts such as aluminum chloride, aluminum nitrate, aluminum sulfate, aluminum polychlorides, ammonium alum, sodium alumiate, potassium aluminate and aluminum hydroxide and hydrates of these salts.

As other methods, there is a method of growing crystals of hydrated alumina by alternately changing the pH values of the acid side and alkali side (as described in JP-A-56-120508), and another method of mixing aluminum oxide obtained from aluminum inorganic salts. The hydrated alumina and the alumina obtained by Bayer's method were subjected to rehydration of alumina (as described in JP-B-4-33728).

Commercially available hydrated alumina is also suitable for use in the inkjet recording media of the present invention. Their examples are listed below, but do not limit the present invention.

Hydrated alumina includes, for example, CATALOID AS-1, CATALOID AS-2, and CATALOID AS-3 (produced by Shokubai Kagaku Kogyo Co., Ltd.), ALUMINA SOL 100, ALUMINA SOL 200, ALUMINA SOL 520 (produced by Nissan Chemical Industries, Ltd.), M-200 (manufactured by Mizusawa Chemical Industries, Ltd), ALUMI SOL 10, ALUMI SOL 20, ALUMI SOL132, ALUMI SOL 132S, ALUMI SOL SH5, ALUMI SOL CSA55, ALUMI SOL SV102 and ALUMI SOL SB52 (manufactured by Kawaken Fine Co. Chemical ).

The aluminum oxide (hereinafter referred to as "alumina") ultrafine particles used in the present invention are preferably γ-type alumina fine particles which are γ-type crystals. γ-type crystals can be divided into γ and δ groups according to crystallography. Fine particles having a delta group crystal form are preferred.

The fine particles of γ-type alumina can be reduced to primary particles with an average particle size of about 10 nanometers, but the primary particles usually form secondary aggregates (hereinafter referred to as "secondary particles"), thereby increasing the particle size to several thousand Nanometer - tens of thousands of nanometers. When γ-type alumina fine particles of such a large particle size are used, the printability and absorbency of the ink-receiving layer are satisfactory, but the ink-receiving layer lacks transparency and tends to generate defects in the coating layer. The average particle size of the primary particles is preferably less than 80 nm. If secondary particles containing primary particles of 80nm or larger are used, brittleness increases and defects occur very frequently in the coating.

In order to obtain a sol of γ-alumina fine particles, the γ-alumina crystals in the form of secondary particles of several thousand nanometers to tens of thousands of nanometers are usually ground into ultrafine particles with an average particle size of 200 nanometers or less by a grinding device. Particles, preferably 100 nanometers or less, said milling apparatus includes beads mill, ultrasonic homogenizer, and high pressure type homogenizer. If the average particle size exceeds 200 nm, the ink absorption will increase, but the coating will be brittle and the transparency will be poor. The grinding device is preferably an ultrasonic homogenizer and a high-pressure homogenizer. In the case of using other grinding methods (such as a ball mill), impurities can easily enter the sol from the grinding container, because γ-type alumina crystals are hard crystals, resulting in transparency. Reduce and generate defects. The γ-type alumina fine particles have excellent ink absorption, excellent printing quality (such as drying and ink fixability), and by making them ultrafine particles, they can make inkjet recording media have excellent transparency, Even if they are contained in the ink receiving layer at a high ratio.

γ-type alumina fine particles are commercially available, for example, alumina C (manufactured by Japan Aerosil Co., Ltd) belongs to the δ group, and AKP-GO15 (manufactured by Sumitomo Chemical Co., Ltd) belongs to the γ group.

Water-soluble or water-insoluble polymers can be used as the binder for the inorganic ultrafine particles in the present invention. The polymer used in the present invention as a constituent component of the ink-receiving layer has an affinity for ink. As examples of water-soluble polymers, mention may be made of cellulose binders (such as methylcellulose, methylhydroxyethylcellulose, methylhydroxypropylcellulose, and hydroxyethylcellulose), natural polymer resins and its derivatives (such as starch and its modified products, gelatin and its modified products, casein, pullulan, gum arabic and albumin), polyvinyl alcohol and its modified products, latex or emulsion (such as styrene - Butadiene copolymers, styrene-acrylate copolymers, methyl methacrylate-butadiene copolymers and ethylene-vinyl acetate copolymers, vinyl polymers (such as polyacrylamide and polyvinylpyrrolidone) , polyethyleneimine, polypropylene glycol, polyethylene glycol, maleic anhydride or a copolymer thereof, etc. Polyvinyl alcohol is preferred.

As the water-insoluble polymer, a water-insoluble binder soluble in alcohol such as a mixed solvent of ethanol and 2-propanol or alcohol and water is preferable because the dispersion of alumina will be stabilized. Examples of these water-insoluble binders are vinylpyrrolidone/vinyl acetate copolymers and acetal resins such as polyvinyl butyral and polyvinyl formal. Acetal resins with an acetalization degree of 5 to 20 mol% are preferred because they can contain a certain amount of water and can facilitate dispersion of inorganic ultrafine particles.

These polymers can be used alone or in combination, based on the inorganic ultrafine particles, in an amount of 2-50% by weight, preferably 5-30% by weight. If the added amount is less than the above range, the coating strength will decrease, and if it exceeds the above range, the ink absorbency will decrease.

To apply the coating solution, various coating methods can be used, such as E-bar coating, curtain coating, straddle sump coating, extrusion coating, roller coating, air knife coating, gravure Coating and rod barcoating.

In the present invention, the structure of the ink-receiving layer may be a single-layer structure or a layered structure. In the case of a layered structure, all layers may comprise the same composition or each layer may comprise a different composition.

In terms of solid content, the coating amount of the ink-receiving layer comprising inorganic ultrafine particles must be not less than 5 g/m ² , and in order to obtain a higher effect of the present invention, it is preferably 10-30 g/m ² , especially preferably 10 g/m 2 . -20 g/ ^m2 . Although it depends on the void content, the thickness is preferably 10-30 micrometers.

In addition, a backcoat can be located on the side of the support opposite to the side having the ink-receiving layer, which can resist curling by balancing the expansion and contraction of the primer and ink-receiving layers.

When a backcoat is provided, the thickness of this layer is preferably 5-30 micrometers, and the backcoat more preferably comprises inorganic pigments and/or spherical organic pigments.

The drying method after coating is not limited, and generally known methods can be used. For example, one method involves feeding the coated carrier into a heating vessel through which heated air generated by a heat source is passed, and another method involves passing the coated carrier through a nearby heat source, such as a heater.

Also, the coating solution used to form the ink-receiving layer comprising inorganic ultrafine particles and optionally a binder may contain various known additives such as surfactants, inorganic pigments, organic dyes, color pigments, ink fixing Agents (cationic resins), UV absorbers, antioxidants, dispersants for pigments, defoamers, leveling agents, preservatives, optical brighteners, viscosity stabilizers, pH regulators and hardeners.

According to the present invention, by forming undercoat (it comprises alkaline earth metal salt and the binder that content is 0.05-0.8 times of alkaline earth metal salt weight) or undercoat (it comprises alkaline earth metal salt, organic pigment and binder) , and an ink-receiving layer comprising inorganic ultrafine particles on the undercoat layer, an inkjet recording medium having high gloss, high ink absorption and excellent image color and high adhesion of the ink-receiving layer can be produced. It is not clear why the above combinations give high gloss and high absorbency. In order to obtain high gloss, when the ink receiving layer is applied, the ink receiving layer must not penetrate into the base coat, and the surface is formed with a high smoothness by leveling or drying the surface. In order to obtain high absorbency, not only the ink receiving layer but also the undercoat must contribute to the absorbency, but if the absorbency of the undercoat is too high, there will be such a contradiction that when coating the ink receiving layer The coating solution for the ink-receiving layer penetrates into the undercoat layer, with the result that high gloss cannot be obtained.

It is considered that, in the present invention, when the pigment in the primer layer is an alkaline earth metal salt, the coating solution used for the ink receiving layer is acidic, and the acid and alkaline earth metal salt in the ink receiving layer will be accepted in the ink receiving layer. layer, therefore, the inorganic ultrafine particles cannot penetrate into the undercoat layer to form an interface, and it is also believed that the alkaline earth metal salt on the interface or in the undercoat layer is due to the ink receiving layer during or after drying. The water or acid in the ink gradually dissolves or changes, forming an absorption path, so that the ink absorption of the primer layer is associated with the voids of the ink receiving layer, thereby improving absorption. Therefore, the coating solution for the ink receiving layer is preferably in the acidic region. The pH of the coating solution for the ink receiving layer is preferably not higher than 5.0, particularly preferably not higher than 4.0. If the pH of the coating solution for the ink-receiving layer exceeds 5.0, the interaction with the alkaline earth metal salt becomes weak and ink absorption decreases. If the paint solution is in the alkaline region, it will be difficult to amplify the effect.

The support used in the present invention is not particularly limited as long as the undercoat layer and the ink receiving layer can be coated thereon, but is preferably a paper support. The pulp constituting the paper preferred in the present invention includes one or more of natural pulp, recycled pulp, synthetic pulp, and the like. As natural pulps it is possible to use these pulps, which are usually used to make paper, i.e. bleached chemical pulps (such as softwood kraft pulp, hardwood kraft pulp, softwood sulphite pulp and hardwood sulphite pulp. In addition, high whiteness can also be used Also, non-wood pulps such as vegetable fibers (such as straw, Spanish grass, bagasse and kenaf), bast fibers (such as hemp, paperwood, ganpi (Diplomorha sikokiana Honda) and mitsumata (Edgeworthia papyrifera Sieb.et .Zucc) and cotton. Among them, the most preferred are bleached chemical pulps, such as softwood kraft pulp, hardwood kraft pulp, softwood sulfite pulp and hardwood sulfite pulp, which are most widely used in the industry.

Pulp is usually beaten with a beater (such as a double-disc beater) to improve the suitability and various paper properties (such as strength, smoothness, and uniformity of texture) in the paper manufacturing process. The degree of beating is selected according to the common range of 250-550 ml of the Canadian standard beating degree.

The beaten pulp is made into paper through a paper machine (such as a fourdrinier paper machine, an improved double fourdrinier paper machine, and a cylinder paper machine). In this case, in the present invention, all additives customary in papermaking, such as dispersing aids for pulp, drying enhancers, wetting enhancers, fillers, sizing agents and fixatives, can be added. Furthermore, pH adjusters, dyes, colored pigments, optical brighteners, etc. may also be added, if necessary.

Dispersion aids include polyethylene oxide, polyacrylamide, and hibiscus; enhancers include anionic enhancers (such as vegetable gum, starch, and carboxy-modified polyvinyl alcohol) and cationic enhancers (such as cationized starch, cationic polyacrylamide and polyamide-polyamine-epichlorohydrin resins); filler materials include clay, kaolin, talc, calcium bicarbonate, precipitated calcium carbonate, barium sulfate, titanium oxide, aluminum hydroxide, and magnesium hydroxide; Gum agents include higher fatty acid salts, rosin, rosin derivatives (such as maleated rosin, dialkylketene dimer, alkenyl or alkyl succinate, epoxidized fatty acid amides and polysaccharide esters; fixatives include Polyvalent metal salts (such as aluminum sulfate and aluminum chloride) and cationic polymers (such as cationized starch, polyamide-polyamine-epichlorohydrin resin); pH adjusters include hydrochloric acid, sodium hydroxide and sodium carbonate.

In addition, the paper support used in the present invention is preferably processed by tank sizing, size printing, gate roll coater, film transfer coater, etc. with a liquid containing various additives (including water-soluble polymer additives). coated.

The above-mentioned water-soluble polymer additives include water-soluble polymer binders (such as starch, starch derivatives (such as cationized starch, oxidized starch, etherified starch and phosphated starch), polyvinyl alcohol, polyvinyl alcohol derivatives ( Such as carboxy-modified polyvinyl alcohol), cellulose derivatives (such as carboxymethyl cellulose, hydroxymethyl cellulose, hydroxyethyl cellulose and cellulose sulfate), water-soluble natural polymers (such as gelatin, casein and soybean protein), water-soluble polymers (such as sodium polyacrylate, styrene-maleic anhydride copolymer sodium salt, sodium polystyrene sulfonate and maleic anhydride resin), and thermosetting synthetic resins (such as melamine resin and urea-formaldehyde resin ). In addition, as sizing agents, mention may be made of petroleum resin emulsions, ammonium salts of styrene-maleic anhydride copolymer alkyl esters, alkyl ketene dimer emulsions and styrene-butadiene copolymers, ethylene - Dispersions of vinyl acetate copolymer, polyethylene and polyvinylidene chloride. Other additives include inorganic electrolytes (such as sodium chloride, calcium chloride and Glauber's salt) as antistatic agents; glycerin and polyethylene glycol as hygroscopic materials Clay, kaolin, talc, barium sulfate and titanium oxide are used as pigments; hydrochloric acid, sodium hydroxide and sodium carbonate are used as pH regulators. Moreover, additives (such as dyes, optical brighteners, antioxidants and UV absorbers) can also be used Use in combination.

The paper carrier used in the present invention preferably has a high surface smoothness obtained by pressing with a calender or other means during or after papermaking. An especially preferred paper support has a Beck smoothness of not less than 50 seconds, particularly preferably not less than 100 seconds (measured according to JISP-8119). The basis weight of the paper support is preferably 70-300 g/ ^m2 , more preferably 150-300 g/ ^m2 . A suitable density is not less than 0.90 g/ ^cm3 . Moreover, the water absorption measured using the Cobb method specified in JIS-P-8140 (contact time: 30 seconds) should not exceed 25 g/ ^m2 , measured using the Gurley paper air permeability tester specified in JIS-P-8117 The air permeability should not be less than 100 sec/100 ml.

Where the paper support used in the present invention has a density of less than 0.90 g/ ^cm3 , the paper support preferably contains a wet-out enhancer.

In addition, when a paper carrier is used in the present invention, a barrier layer containing pigments and binders can be placed between the base paper and the base coat to prevent ink solvents from reaching the base paper, resulting in less surface waviness and a smoother surface after the image is recorded. An inkjet recording medium with an attractive appearance.

Example

The invention is illustrated by the following examples, which should not be construed as limiting the invention. All parts and % are by weight.

<preparation carrier>

A mixture (1:1) of hardwood bleached kraft pulp (LBKP, brightness: 90%) and softwood bleached sulfite pulp (NBSP, brightness: 90%) was beaten until the Canadian standard beating degree reached 300 ml, thereby preparing pulp. In the pulp obtained, add alkyl ketene dimer as a sizing agent (based on pulp, its content is 0.5% by weight), polyacrylamide as a reinforcing agent (based on pulp, its content is 1.0% by weight), cationic Starch (2.0% by weight based on pulp) and polyamide-epichlorohydrin resin (0.5% by weight based on pulp) were then diluted with water to obtain a 1% slurry. The resulting slurry is subjected to a papermaking process using a fourdrinier paper machine to make paper, which is subjected to 3 steps of wet pressing in a wet state, then processed with smooth rolls, and then subjected to 2 steps of stretching and pressing during the subsequent drying process. Afterwards, during the drying process, the paper was coated with a size press solution of 20 g/m ² , which contained 5% by weight of carboxy-modified polyvinyl alcohol, and then dried to finally obtain The water content in the base paper reaches an absolute dry water content of 8% by weight. Next, mechanical calendering was performed to produce a paper having a basis weight of 170 g/m ² , thereby obtaining a paper carrier. The Beck smoothness of this paper support was 110 seconds.

<Coating solution 1a-1h for preparation of undercoat layer>

100 parts of precipitated calcium carbonate (TAMAPEARL 222H produced by Okutama Kogyo Co., Ltd.) as an alkaline earth metal salt and 3 parts, 5 parts, 10 parts, 15 parts, 20 parts, 40 parts, 80 parts or 100 parts as a binder Parts (in terms of solid content) of styrene-butadiene copolymer latex (LACSTAR DS226 produced by Dainippon Ink & Chemicals Inc.) were mixed with water to prepare a coating solution 1a-1h with a solid content of 45% for the undercoat layer.

<Coating solution 2 for preparation of undercoat layer>

100 parts of heavy calcium carbonate (CARBITAL 90 produced by ECC International Co., Ltd.) as an alkaline earth metal salt and 20 parts (in terms of solid content) of styrene-butadiene copolymer latex (Dainippon Ink & Co., Ltd.) as a binder LACSTAR DS226 produced by Chemicals Inc.) was mixed with water to prepare a coating solution 2 with a solids content of 45% for an undercoat layer.

<Coating solution 3 for preparation of undercoat layer>

100 parts of magnesium carbonate (spherical magnesium carbonate produced by Kamishima Kagaku Kogyo Co., Ltd) as an alkaline earth metal salt and 20 parts (in terms of solid content) of styrene-butadiene copolymer latex (Dainippon Ink & Chemicals Inc. LACSTAR DS226) was mixed with water to prepare a coating solution 3 with a solids content of 45% for the base coat.

<Coating solution 4 for preparation of undercoat layer>

100 parts of barium sulfate (precipitated barium sulfate D-2 produced by Baryte Kogyo Co., Ltd) as alkaline earth metal salt and 20 parts (in terms of solid content) of styrene-butadiene copolymer latex as binder (LACSTAR DS226 produced by Dainippon Ink & Chemicals Inc.) was mixed with water to prepare a coating solution 4 having a solids content of 45% for an undercoat layer.

<Coating solution 5 for preparation of undercoat layer>

100 parts of synthetic amorphous silica (FINESIL X37B produced by Tokuyama Co., Ltd.) as an alkaline earth metal salt and 20 parts of polyvinyl alcohol (PVA117 produced by Kuraray Co., Ltd.) as a binder were mixed with water to prepare Coating solution 5 with a high solids content of 20% for the base coat.

<Coating solution 6a for preparation of undercoat layer>

80 parts of precipitated calcium carbonate (TAMAPEARL 222H produced by Okutama Kogyo Co., Ltd.), 20 parts of hollow organic pigment (ROPAQUE HP-91 produced by Rohm & Haas Co.; average particle diameter: 1.0 μm, average void) as alkaline earth metal salt Content: 50%) and 20 parts (by solid content) of styrene-butadiene copolymer latex (LACSTAR DS226 produced by Dainippon Ink & Chemicals Inc.) as a binder are mixed with water to prepare the base coat coating solution 6a with a solids content of 45%.

<Coating solution 6b for preparation of undercoat layer>

50 parts of precipitated calcium carbonate (TAMAPEARL 222H produced by Okutama Kogyo Co., Ltd.), 50 parts of hollow organic pigment (ROPAQUE HP-91 produced by Rohm & Haas Co.; average particle diameter: 1.0 μm, average void) as alkaline earth metal salt Content: 50%) and 20 parts (by solid content) of styrene-butadiene copolymer latex (LACSTAR DS226 produced by Dainippon Ink & Chemicals Inc.) as a binder are mixed with water to prepare the base coat Coating solution 6b with 45% solids content.

<Coating solution 6c for preparation of undercoat layer>

20 parts of precipitated calcium carbonate (TAMAPEARL 222H produced by Okutama Kogyo Co., Ltd.), 80 parts of hollow organic pigment (ROPAQUE HP-91 produced by Rohm & Haas Co.; average particle diameter: 1.0 μm, average void) as alkaline earth metal salt Content: 50%) and styrene-butadiene copolymer latex (LACSTAR DS226 produced by Dainippon Ink & Chemicals Inc.) of 20 parts (in terms of solid content) as a binder are mixed with water to prepare the base coat Coating solution 6c with 45% solids content.

<Coating solution 7 for preparation of undercoat layer>

80 parts of heavy calcium carbonate (CARBITAL 90 produced by ECC International Co., Ltd.), 20 parts of hollow organic pigment (ROPAQUE HP-91 produced by Rohm & Haas Co.; average particle diameter: 1.0 μm, average void space) as alkaline earth metal salt Content: 50%) and styrene-butadiene copolymer latex (LACSTAR DS226 produced by Dainippon Ink & Chemicals Inc.) of 20 parts (in terms of solid content) as a binder are mixed with water to prepare the base coat Coating Solution 7 with 45% solids content.

<Coating solution 8 for preparation of undercoat layer>

80 parts of magnesium carbonate as alkaline earth metal salt (spherical magnesium carbonate produced by Kamishima Kagaku Kogyo Co., Ltd), 20 parts of hollow organic pigment (ROPAQUE HP-91 produced by Rohm & Haas Co.; average particle diameter: 1.0 μm, average Void content: 50%) and 20 parts (in terms of solid content) of styrene-butadiene copolymer latex (LACSTAR DS226 produced by Dainippon Ink & Chemicals Inc.) as a binder were mixed with water to prepare an undercoat layer Coating solution 8 with a solids content of 45% was used.

<Coating solution 9 for preparation of undercoat layer>

80 parts of kaolin (UW90 produced by Engelhard Co., Ltd) as alkaline earth metal salt, 20 parts of hollow organic pigment (ROPAQUE HP-91 produced by Rohm & Haas Co.; average particle diameter: 1.0 μm, average void content: 50% ) and 20 parts (based on solid content) of styrene-butadiene copolymer latex (LACSTAR DS226 produced by Dainippon Ink & Chemicals Inc.) as a binder are mixed with water to prepare a primer with 45% Solids content of coating solution 9.

<Coating solution 10 for preparation of undercoat layer>

100 parts of hollow organic pigment (ROPAQUE HP-91 produced by Rohm & Haas Co.; average particle size: 1.0 μm, average void content: 50%) and 20 parts (in terms of solid content) of styrene as a binder- Butadiene copolymer latex (LACSTAR DS226 produced by Dainippon Ink & Chemicals Inc.) was mixed with water to prepare a coating solution 10 having a solid content of 45% for an undercoat layer.

<Coating solution 11 for preparation of undercoat layer>

80 parts of precipitated calcium carbonate (TAMAPEARL 222H produced by Okutama Kogyo Co., Ltd.), 20 parts of hollow organic pigment (ROPAQUE OP-84J produced by Rohm & Haas Co.; average particle diameter: 0.55 μm, average void) as alkaline earth metal salt Content: 25%) and 20 parts (in terms of solid content) of styrene-butadiene copolymer latex (LACSTAR DS226 produced by Dainippon Ink & Chemicals Inc.) as a binder are mixed with water to prepare the primer layer Coating solution 11 having a solids content of 45%.

<Coating solution 12 for preparation of undercoat layer>

80 parts of precipitated calcium carbonate (TAMAPEARL 222H produced by Okutama Kogyo Co., Ltd.), 20 parts of dense organic pigment (L8801 produced by Asahi Kasei Kogyo K.K.; average particle diameter: 0.5 μm) as a binder, and 20 parts (in terms of solid content) of styrene-butadiene copolymer latex (LACSTAR DS226 produced by Dainippon Ink & Chemicals Inc.) was mixed with water to prepare a coating solution 12 having a solid content of 45% for an undercoat layer.

<Coating solution 13 for preparation of undercoat layer>

80 parts of precipitated calcium carbonate (TAMAPEARL 222H produced by Okutama Kogyo Co., Ltd.), 20 parts of dense organic pigment (ARTPEARLF-4P produced by Negami Kogyo Co., Ltd.; average particle diameter: 2.1 μm) as an alkaline earth metal salt, and as 20 parts (in terms of solid content) of the styrene-butadiene copolymer latex (LACSTAR DS226 produced by Dainippon Ink & Chemicals Inc.) of the adhesive was mixed with water to prepare a primer with a solid content of 45%. Coating solution 13.

<Coating solution 14 for preparation of undercoat layer>

80 parts of precipitated calcium carbonate (TAMAPEARL 222H produced by Okutama Kogyo Co., Ltd.), 20 parts of bowl-shaped compact organic pigment (V2005 produced by Nippon Zeon Co., Ltd.; average particle diameter: 0.8 μm) as an alkaline earth metal salt, and as 20 parts (in terms of solid content) of styrene-butadiene copolymer latex (LACSTAR DS226 produced by Dainippon Ink & Chemicals Inc.) of the adhesive was mixed with water to prepare a primer with a solid content of 45%. Coating solution 14.

<Coating solution 15a for preparing undercoat layer>

80 parts of precipitated calcium carbonate (TAMAPEARL 222H produced by Okutama Kogyo Co., Ltd.), 5 parts of hollow organic pigment (ROPAQUE OP-84J produced by Rohm & Haas Co.; average particle diameter: 0.55 μm, average void) as alkaline earth metal salt Content: 25%), 15 parts of dense organic pigment (L8801 produced by Asahi KaseiKogyo K.K.; average particle diameter: 0.5 micron) and 20 parts (in terms of solid content) of styrene-butadiene copolymer latex as a binder (LACSTAR DS226 produced by Dainippon Ink & Chemicals Inc.) was mixed with water to prepare a coating solution 15a having a solids content of 45% for an undercoat layer.

<Coating solution 15b for preparation of undercoat layer>

80 parts of precipitated calcium carbonate (TAMAPEARL 222H produced by Okutama Kogyo Co., Ltd.), 10 parts of hollow organic pigment (ROPAQUE OP-84J produced by Rohm & Haas Co.; average particle diameter: 0.55 μm, average void) as alkaline earth metal salt Content: 25%), 10 parts of dense organic pigment (L8801 produced by AsahiKasei Kogyo K.K.; average particle diameter: 0.5 micrometers) and 20 parts (in terms of solid content) of styrene-butadiene copolymer latex as a binder (LACSTAR DS226 produced by Dainippon Ink & Chemicals Inc.) was mixed with water to prepare a coating solution 15b having a solids content of 45% for an undercoat layer.

<Coating Solution 15c for Undercoating>

80 parts of precipitated calcium carbonate (TAMAPEARL 222H produced by Okutama Kogyo Co., Ltd.), 15 parts of hollow organic pigment (ROPAQUE OP-84J produced by Rohm & Haas Co.; average particle size: 0.55 μm, average void) as alkaline earth metal salt Content: 25%), 5 parts of dense organic pigment (L8801 produced by AsahiKasei Kogyo K.K.; average particle diameter: 0.5 micron) and 20 parts (in terms of solid content) of styrene-butadiene copolymer latex as a binder (LACSTAR DS226 produced by Dainippon Ink & Chemicals Inc.) was mixed with water to prepare a coating solution 15c having a solids content of 45% for an undercoat layer.

<Coating solutions 16a to 16d for preparing undercoat layer>

80 parts of precipitated calcium carbonate (TAMAPEARL 222H produced by Okutama Kogyo Co., Ltd.), 20 parts of hollow organic pigment (ROPAQUE HP-91 produced by Rohm & Haas Co.; average particle diameter: 1.0 μm, average void) as alkaline earth metal salt Content: 50%) and 3 parts, 5 parts, 80 parts or 100 parts (based on solid content) of styrene-butadiene copolymer latex (LACSTAR DS226 produced by Dainippon Ink & Chemicals Inc.) as a binder and Water was mixed to prepare coating solutions 16a-16d having a solids content of 45% for the base coat.

<Coating solution 17 for preparation of undercoat layer>

80 parts of precipitated calcium carbonate (TAMAPEARL 222H produced by Okutama Kogyo Co., Ltd.), 20 parts of dense organic pigment (L8999 produced by Asahi Kasei Kogyo K.K.; average particle diameter: 0.2 μm) as an alkaline earth metal salt, and 20 parts (in terms of solid content) of styrene-butadiene copolymer latex (LACSTAR DS226 produced by Dainippon Ink & Chemicals Inc.) was mixed with water to prepare a coating solution 17 having a solid content of 45% for an undercoat layer.

<Coating solution 18 for preparation of undercoat layer>

80 parts of precipitated calcium carbonate (TAMAPEARL 222H produced by Okutama Kogyo Co., Ltd.), 20 parts of dense organic pigment (CHEMIPEARL V-100 produced by Mitsui Chemical Co., Ltd.; average particle diameter: 12.0 μm) as an alkaline earth metal salt, and As a binder, 20 parts (in terms of solid content) of styrene-butadiene copolymer latex (LACSTAR DS226 produced by Dainippon Ink & Chemicals Inc.) was mixed with water to prepare a primer with a solid content of 45%. 18 of the coating solution.

<Coating Solution A for Ink Receiving Layer>

100 g of ultrafine particle silica (AEROSIL 300 produced by Japan Aerosil Co., Ltd) and 3 g of dispersant (SHALLOL DC902P produced by Daiichi Kogyo Seiyaku Co., Ltd) prepared by a gas phase method with a primary particle diameter of 7 nm were stirred Dispersed in 500 g of deionized water to obtain a dispersion with a secondary particle size of 200 nm or less. 100 parts of this ultrafine particle silica dispersion prepared by a gas phase method was mixed with 15 parts of a 10% by weight aqueous solution of polyvinyl alcohol (PVA 105 produced by Kuraray Co., Ltd), and then water was added to prepare the ink receiving layer. A coating solution with a solids content of 15%. The resulting coating solution A for the ink-receiving layer had a pH of 3.8.

<Synthetic hydrated alumina>

A 3 liter reaction vessel was charged with 1200 grams of deionized water and 900 grams of isopropanol and heated to 75°C. An additional 408 g of aluminum isopropoxide was added, hydrolyzed at 75°C for 24 hours, and continued at 95°C for 10 hours. After hydrolysis was complete, 24 g of acetic acid was added, followed by stirring at 95° C. for 48 hours, followed by concentration to a solid content of 15% by weight to obtain a white dispersion of ultrafine particle hydrated alumina. The sol was dried at room temperature and subjected to X-ray diffraction, revealing the pseudoboehmite structure. In addition, the primary particle diameter measured by a transmission electron microscope is 30 nanometers, and the hydrated alumina is a flaky ultrafine particle hydrated alumina with an aspect ratio of 6.0. Furthermore, the average pore diameter, pore volume and BET specific surface area measured by nitrogen absorption and desorption methods were 7.1 nm, 0.65 ml/g and 200 ^m2 /g, respectively.

<Coating Solution B for Ink Receiving Layer>

The above-mentioned dispersion of ultrafine particle hydrated alumina with a concentration of 15% by weight is dispersed using a homomixer so that the secondary particle diameter becomes 400 nm or less, and 100 parts of this hydrated alumina dispersion is mixed with 15 parts of polyethylene A 10% by weight aqueous solution of alcohol (PVA 105 produced by Kuraray Co., Ltd.) was mixed. The resulting mixed liquid was concentrated with an evaporator to a solid content of 15%, thereby obtaining a coating solution B for an ink-receiving layer. The resulting coating solution B for the ink receiving layer had a pH of 4.5.

<Coating Solution C for Ink Receiving Layer>

600 grams of Aerosil Aluminum Oxide C (produced by Japan Aerosil Co., Ltd) with a primary particle diameter of 13 nanometers (it is the γ-type alumina crystal powder of the δ family) is dispersed in 2400 grams with a homomixer as alumina superfine particles In ionized water, such that the secondary particle diameter becomes 100 nm or less, a 20% by weight viscous solution in the form of a slurry is prepared. 100 parts of this dispersion having a concentration of γ-type alumina of 20% by weight was mixed with 30 parts of a 10% by weight aqueous solution of polyvinyl alcohol (PVA 235 produced by Kuraray Co., Ltd). Water was then added to prepare Coating Solution C having a solids content of 15% for the ink-receiving layer. The resulting coating solution C for the ink-receiving layer had a pH of 5.0.

<Coating Solution D for Ink Receiving Layer>

100 g of ultrafine particle silica (AEROSIL 300 produced by Japan Aerosil Co., Ltd) and 3 g of dispersant (SHALLOL DC902P produced by Daiichi Kogyo Seiyaku Co., Ltd) prepared by a gas phase method with a primary particle diameter of 7 nm were stirred Dispersed in 500 g of deionized water to obtain a dispersion with a secondary particle size of 200 nm or less. 100 parts of this ultrafine particle silica dispersion prepared by the gas phase method was mixed with 15 parts of a 10% by weight aqueous solution of polyvinyl alcohol (PVA 105 produced by Kuraray Co., Ltd), and then sodium hydroxide was added to adjust the pH to 5.5. Add water to prepare a coating solution D with a solid content of 15% for the ink receiving layer.

<Coating Solution E for Ink Receiving Layer>

100 parts of synthetic amorphous silica (FINESIL X-37B produced by Tokuyama Co., Ltd; BET specific surface area: 270 ^m2 /g) with a secondary particle diameter of 3.7 μm (it is not the inorganic ultrafine particles of the present invention) , 15 parts of polyvinyl alcohol (PVA 105 produced by Kuraray Co., Ltd) and 20 parts of cationic dye fixing agent (SUMIREZ RESIN 1001 produced by Sumitomo Chemical Co., Ltd) were dissolved in water and mixed to obtain a solid for the ink receiving layer Coating solution E at a concentration of 15%. The pH of the coating solution E for the ink receiving layer was 5.3.

Example 1

The coating solution 1b for the undercoat layer was applied to the above-prepared paper support by an air knife coater at a dry solids content of 15 g/m ² , and the coating was then dried. Next, coating solution A for an ink-receiving layer having a dry solid content of 15 g/ ^m2 was coated with a curtain coater on the obtained undercoat layer, followed by drying to obtain the recording medium of Example 1.

Example 2

The recording medium of Example 2 was prepared in the same manner as in Example 1, except that the coating solution 1c for an undercoat layer was used instead of the coating solution 1b for an undercoat layer in Example 1.

Example 3

The recording medium of Example 3 was prepared in the same manner as in Example 1, except that the coating solution 1d for an undercoat layer was used instead of the coating solution 1b for an undercoat layer in Example 1.

Example 4

The recording medium of Example 4 was prepared in the same manner as in Example 1, except that the coating solution 1e for an undercoat layer was used instead of the coating solution 1b for an undercoat layer in Example 1.

Example 5

The recording medium of Example 5 was prepared in the same manner as in Example 1, except that the coating solution for undercoat layer 1f was used instead of the coating solution for undercoat layer 1b in Example 1.

Example 6

The recording medium of Example 6 was prepared in the same manner as in Example 1, except that 1 g of the coating solution for an undercoat layer was used instead of the coating solution 1 b for an undercoat layer in Example 1.

Comparative example 1

A recording medium of Comparative Example 1 was prepared in the same manner as in Example 1, except that the coating solution for undercoat layer 1a was used instead of the coating solution for undercoat layer 1b in Example 1.

Comparative example 2

A recording medium of Comparative Example 2 was prepared in the same manner as in Example 1, except that the coating solution for an undercoat layer 1h was used instead of the coating solution for an undercoat layer 1b in Example 1.

Example 7

The recording medium of Example 7 was prepared in the same manner as in Example 1, except that the coating solution 2 for an undercoat layer was used instead of the coating solution 1b for an undercoat layer in Example 1.

Example 8

The recording medium of Example 8 was prepared in the same manner as in Example 1, except that the coating solution 3 for an undercoat layer was used instead of the coating solution 1b for an undercoat layer in Example 1.

Example 9

The recording medium of Example 9 was prepared in the same manner as in Example 1, except that the coating solution 4 for an undercoat layer was used instead of the coating solution 1b for an undercoat layer in Example 1.

Comparative example 3

A recording medium of Comparative Example 3 was prepared in the same manner as in Example 1, except that the coating solution 5 for an undercoat layer was used instead of the coating solution 1b for an undercoat layer in Example 1.

Example 10

The recording medium of Example 10 was prepared in the same manner as in Example 3, except that the ink-receiving layer coating solution B was used instead of the ink-receiving layer coating solution A in Example 3.

Example 11

The recording medium of Example 11 was prepared in the same manner as in Example 3, except that the ink-receiving layer coating solution C was used instead of the ink-receiving layer coating solution A in Example 3.

Example 12

The recording medium of Example 12 was prepared in the same manner as in Example 3, except that the ink-receiving layer coating solution D was used instead of the ink-receiving layer coating solution A in Example 3.

Comparative example 4

The recording medium of Comparative Example 4 was prepared in the same manner as in Example 3, except that the ink-receiving layer coating solution E was used instead of the ink-receiving layer coating solution A in Example 3.

Example 13

The recording medium of embodiment 13 was prepared in the same manner as in embodiment 3, except that after coating and drying the undercoat, the undercoat was subjected to thermal calendering (temperature: 100° C., nip pressure: 150 kg/cm ).

Example 14

The recording medium of embodiment 14 was prepared in the same manner as in embodiment 10, except that after coating and drying the undercoat, the undercoat was subjected to thermal calendering (temperature: 100° C., nip pressure: 150 kg/cm ), and the pH of the coating solution B for the ink-receiving layer was adjusted to 4.0 with hydrochloric acid.

Example 15

The recording medium of Example 15 was prepared in the same manner as in Example 1, except that the coating solution 6a for an undercoat layer was used instead of the coating solution 1b for an undercoat layer in Example 1.

Example 16

The recording medium of Example 16 was prepared in the same manner as in Example 1, except that the coating solution 6b for an undercoat layer was used instead of the coating solution 1b for an undercoat layer in Example 1.

Example 17

The recording medium of Example 17 was prepared in the same manner as in Example 1, except that the coating solution 6c for an undercoat layer was used instead of the coating solution 1b for an undercoat layer in Example 1.

Example 18

The recording medium of Example 18 was prepared in the same manner as in Example 1, except that the coating solution 7 for an undercoat layer was used instead of the coating solution 1b for an undercoat layer in Example 1.

Example 19

The recording medium of Example 19 was prepared in the same manner as in Example 1, except that the coating solution 8 for an undercoat layer was used instead of the coating solution 1b for an undercoat layer in Example 1.

Example 20

The recording medium of Example 20 was prepared in the same manner as in Example 1, except that the coating solution 11 for an undercoat layer was used instead of the coating solution 1b for an undercoat layer in Example 1.

Example 21

The recording medium of Example 21 was prepared in the same manner as in Example 1, except that the coating solution 12 for an undercoat layer was used instead of the coating solution 1b for an undercoat layer in Example 1.

Example 22

The recording medium of Example 22 was prepared in the same manner as in Example 1, except that the coating solution 13 for an undercoat layer was used instead of the coating solution 1b for an undercoat layer in Example 1.

Example 23

The recording medium of Example 23 was prepared in the same manner as in Example 1, except that the coating solution 14 for an undercoat layer was used instead of the coating solution 1b for an undercoat layer in Example 1.

Example 24

The recording medium of Example 24 was prepared in the same manner as in Example 1, except that the coating solution for undercoat layer 15a was used instead of the coating solution for undercoat layer 1b in Example 1.

Example 25

The recording medium of Example 25 was prepared in the same manner as in Example 1, except that the coating solution 15b for an undercoat layer was used instead of the coating solution 1b for an undercoat layer in Example 1.

Example 26

The recording medium of Example 26 was prepared in the same manner as in Example 1, except that the coating solution for undercoat layer 15c was used instead of the coating solution for undercoat layer 1b in Example 1.

Example 27

The recording medium of embodiment 27 was prepared in the same manner as in embodiment 15, except that after coating and drying the undercoat, the undercoat was subjected to thermal calendering (temperature: 100° C., nip pressure: 150 kg/cm ).

Example 28

The recording medium of embodiment 28 was prepared in the same manner as in embodiment 23, except that after coating and drying the undercoat, the undercoat was subjected to thermal calendering (temperature: 100° C., nip pressure: 150 kg/cm ).

Example 29

The recording medium of embodiment 29 is prepared in the same manner as in embodiment 24, except that after coating and drying the undercoat, the undercoat is subjected to thermal calendering (temperature: 100° C., nip pressure: 150 kg/cm ).

Example 30

The recording medium of Example 30 was prepared in the same manner as in Example 15, except that the ink-receiving layer coating solution B was used instead of the ink-receiving layer coating solution A in Example 15.

Example 31

The recording medium of Example 31 was prepared in the same manner as in Example 15, except that the ink-receiving layer coating solution C was used instead of the ink-receiving layer coating solution A in Example 15.

Example 32

The recording medium of embodiment 32 was prepared in the same manner as in embodiment 30, except that after coating and drying the undercoat, the undercoat was subjected to thermal calendering (temperature: 100° C., nip pressure: 150 kg/cm ).

Example 33

The recording medium of embodiment 33 is prepared in the same manner as in embodiment 31, except that after coating and drying the undercoat, the undercoat is subjected to thermal calendering (temperature: 100° C., nip pressure: 150 kg/cm ).

Example 34

The recording medium of Example 34 was prepared in the same manner as in Example 15, except that the coating solution 16a for an undercoat layer was used instead of the coating solution 6a for an undercoat layer in Example 15.

Example 35

The recording medium of Example 35 was prepared in the same manner as in Example 15, except that the coating solution 16b for an undercoat layer was used instead of the coating solution 6a for an undercoat layer in Example 15.

Example 36

The recording medium of Example 36 was prepared in the same manner as in Example 15, except that the coating solution 16c for an undercoat layer was used instead of the coating solution 6a for an undercoat layer in Example 15.

Example 37

The recording medium of Example 37 was prepared in the same manner as in Example 15, except that the coating solution 16d for an undercoat layer was used instead of the coating solution 6a for an undercoat layer in Example 15.

Example 38

The recording medium of Example 38 was prepared in the same manner as in Example 15, except that the coating solution 17 for an undercoat layer was used instead of the coating solution 6a for an undercoat layer in Example 15.

Example 39

The recording medium of Example 39 was prepared in the same manner as in Example 15, except that the coating solution 18 for an undercoat layer was used instead of the coating solution 6a for an undercoat layer in Example 15.

Example 40

The recording medium of Example 40 was prepared in the same manner as in Example 15, except that the ink-receiving layer coating solution D was used instead of the ink-receiving layer coating solution A in Example 15.

Comparative Example 5

A recording medium of Comparative Example 5 was prepared in the same manner as in Example 15, except that coating solution 9 for an undercoat layer was used instead of coating solution 6a for an undercoat layer in Example 15.

Comparative example 6

The recording medium of Comparative Example 6 was prepared in the same manner as in Example 15, except that the coating solution 10 for an undercoat layer was used instead of the coating solution 6a for an undercoat layer in Example 15.

Comparative Example 7

The recording medium of Comparative Example 7 was prepared in the same manner as in Example 15, except that the ink-receiving layer coating solution E was used instead of the ink-receiving layer coating solution A in Example 15.

Evaluation was performed by the following methods. The results are shown in Table 1-3.

<Evaluation of ink absorbency>

The cyan ink, the magenta ink, and the yellow ink were printed on the sample in a rectangular pattern with overlapping colors using an inkjet recording apparatus PM9000 (manufactured by Epson Co., Ltd). When the amount of various overlapping inks is 100%, this is called 300%, when the amount of each ink is 90%, this is called 270%, so that 240%, 210%, 180% and 150% rectangular graphics. The state of ink bleeding at the boundary portion between the printed figure and the unprinted portion was evaluated with the naked eye, and rated by the following criteria.

5: No overflow was seen at 300% printing.

4: No overflow was seen at 270% printing.

3: No overflow was seen at 240% printing.

2: No overflow was seen at 210% printing.

1: No overflow was seen at 180% printing.

Level 5-3 indicates satisfactory ink absorbency, and even in the case of level 2, there is no problem in actual use.

<Evaluation of image color>

Solid printing with magenta ink and cyan ink using Canon BJC-420J. The color of the printed image was evaluated with the naked eye, and the results were rated by the following criteria.

5: Image color is good and vivid.

4: Image color is good.

3: The color of the image is mediocre.

2: The color of the image is slightly dull.

1: The color of the image is dull and dull.

Levels 5-3 indicate satisfactory image colors, and even in the case of level 2, there is no problem in practical use.

<Gloss evaluation of unprinted white paper>

The state of gloss of the unprinted recording surface can be evaluated with the naked eye.

5: The sense of gloss is very high.

4: The sense of gloss is high.

3: Glossiness is average.

2: Glossiness is poor.

1: Glossiness is poor.

Levels 5-3 indicate satisfactory white paper gloss, and even in the case of level 2, there is no problem in actual use.

<Adhesion evaluation>

The coating layer on the recording surface of the recording medium was cut with a cutter at 5 mm cross intervals, then the adhesive tape was applied to the cut surface, and peeled off, and the number of peeled squares of the ink receiving layer per 100 squares (square) was counted . Adhesion was evaluated based on the number of peeled squares.

4: The number of stripped squares is 0.

3: The number of stripped squares is less than 10.

2: The number of stripped squares is 10-30.

1: The number of stripped squares is 31 or more.

Levels 3 and 4 indicate satisfactory adhesion, and even in the case of 2, there is no problem in actual use.

<Measurement of smoothness>

The smoothness of the unprinted recording surface was measured according to JIS-P-8119 using a Beck tester. The unit of measurement is seconds, with higher values increasing smoothness.

Table 1 recording medium ink absorption image color glossy white paper adhesiveness Comparative example 1 5 4 2 1 Example 1 5 4 3 2 Example 2 5 4 3 3 Example 3 5 4 3 3 Example 4 5 4 3 3 Example 5 4 4 3 3 Example 6 2 4 3 3 Comparative example 2 1 4 3 3 Example 7 4 4 3 3 Example 8 5 4 3 3 Example 9 3 4 2 3 Comparative example 3 1 4 2 2 Example 10 5 4 3 3 Example 11 4 4 3 3 Example 12 2 4 3 3 Comparative example 4 4 1 1 3 Example 13 5 4 4 3 Example 14 5 4 4 3

Table 2 recording medium ink absorption image color glossy white paper adhesiveness Smoothness (seconds) Example 15 5 5 4 3 216 Example 16 4 5 4 3 220 Example 17 3 4 4 3 238 Example 18 4 5 4 3 202 Example 19 4 5 4 3 210 Example 20 5 5 4 3 213 Example 21 5 4 4 3 207 Example 22 5 4 4 3 208 Example 23 5 5 4 3 212 Example 24 5 5 4 3 218 Example 25 5 5 4 3 218 Example 26 5 5 4 3 220 Example 27 5 5 5 3 277 Example 28 5 5 5 3 265 Example 29 5 5 5 3 280

table 3 recording medium ink absorption image color glossy white paper adhesiveness Smoothness (seconds) Example 30 4 5 4 3 206 Example 31 4 5 4 3 202 Example 32 4 5 5 3 253 Example 33 4 5 5 3 256 Example 34 5 4 4 2 204 Example 35 5 5 4 3 208 Example 36 4 4 4 4 221 Example 37 3 3 4 4 228 Example 38 3 3 4 3 213 Example 39 3 3 5 3 215 Example 40 3 4 4 3 214 Comparative Example 5 1 1 4 3 231 Comparative Example 6 1 2 4 3 278 Comparative Example 7 2 2 1 3 82

According to Table 1, we can see the following results. In Examples 1-14, in which the ink-receiving layer comprising inorganic ultrafine particles is located on an undercoat layer comprising an alkaline earth metal salt and a binder in an amount of 0.05 to 0.8 times the weight of the alkaline earth metal salt, we obtain excellent Inkjet recording media with well-balanced white paper visible gloss, image color and ink absorption. On the other hand, when the undercoat layer did not contain an alkaline earth metal salt (Comparative Example 3), the recording medium had poor gloss and absorbency; even if the undercoat layer contained an alkaline earth metal salt, when the weight ratio of the binder was less than 0.05 (Comparative Example In case of Example 1), the adhesion is also poor, and when the weight ratio of the binder exceeds 0.8 (Comparative Example 2), the absorption is poor, which is not preferable in practical use. Furthermore, when the pigment in the ink-receiving layer was not inorganic ultrafine particles (Comparative Example 4), the gloss and color of the recording medium were poor. The recording medium of Example 12 (in which the pH of the coating solution for the ink-receiving layer exceeded 5.0) was slightly inferior in ink absorbency to Examples 3, 10 and 11 (in which the pH was not more than 5.0). The visual gloss of the white paper can be further improved on the recording media when the heat calendering treatment (Examples 13 and 14) is applied after the primer coating, which is the preferred method.

From Tables 2 and 3, the following results were obtained. In Examples 15-26, 30, 31 and 34-40, wherein the ink-receiving layer comprising inorganic ultrafine particles is located on the undercoat layer comprising alkaline earth metal salt, organic pigment and binder, the resulting recording medium has excellent and a well-balanced white paper gloss, image color, ink absorption, smoothness and adhesion. However, when the weight ratio of the binder relative to the total solid content was less than 0.05 times (Example 34), the recording medium had excellent ink absorption, but had slightly poor adhesion, and when it exceeded 0.8 times (Example 37 ), the recording medium has excellent adhesiveness but slightly poor ink absorbency, so a weight ratio of 0.05-0.8 times is preferable. When the particle size of the organic pigment is less than 0.3 micrometers (Example 38), the recording medium has slightly poor ink absorption and image color, and when it exceeds 10 micrometers (Example 39), the recording medium has excellent whiteness. Paper is glossy, but has slightly poor ink absorption and image color, so the particle size of the organic pigment is preferably 0.3-10 microns. When the pH of the coating solution for the ink-receiving layer is higher than 5 (Example 40), the ink absorbency is somewhat poor, and the pH is preferably not higher than 5. However, when the undercoat layer contains salts other than alkaline earth metal salts (Comparative Example 5), ink absorbency and image color are deteriorated even when the undercoat layer contains alkaline earth metal salts, if it does not contain organic pigments (Example 4), the gloss of the white paper was slightly deteriorated, and on the other hand, when the undercoat layer contained only organic pigments (Comparative Example 6), the ink absorption was poor. In addition, even when the undercoat layer contains salts other than alkaline earth metal salts and organic pigments, if the pigments in the ink receiving layer are not inorganic ultrafine particles (Comparative Example 7), the gloss and smoothness of white paper are low , the ink absorption and image color are poor. Also, when the heat calendering treatment (Examples 27-29, 32 and 33) was performed after the primer coating, the gloss and smoothness of the white paper were further improved, so this is the preferred manufacturing method.

industrial application

As described above, the present invention can provide an inkjet recording medium having high gloss, excellent ink absorption and image color, and the inkjet recording medium has no adhesion problem of a coating layer.

Claims (21)

1. An inkjet recording medium comprising a support, an undercoat layer on the support and an ink receiving layer formed by coating a coating solution comprising inorganic ultrafine particles on the undercoat layer, wherein the undercoat layer comprises alkaline earth Metal salts and binders, the pH of the coating solution for the ink receiving layer is not higher than 5.0. 2. The inkjet recording medium according to claim 1, wherein the undercoat layer contains 0.05-0.8 times the weight of the alkaline earth metal salt as a binder. 3. The inkjet recording medium according to claim 1 or 2, wherein the alkaline earth metal is calcium or magnesium. 4. The inkjet recording medium according to claim 1 or 2, characterized in that the alkaline earth metal salt is a carbonate. 5. The inkjet recording medium according to claim 1 or 2, wherein the inorganic ultrafine particles are amorphous synthetic silica or alumina compounds prepared by a gas phase method. 6. The inkjet recording medium according to claim 1 or 2, characterized in that the undercoat layer contains 0.05-0.4 times the weight of the alkaline earth metal salt as a binder. 7. The inkjet recording medium according to claim 1, wherein the undercoat layer contains an organic pigment. 8. The ink jet recording medium according to claim 7, wherein the alkaline earth metal is calcium or magnesium. 9. The ink jet recording medium of claim 7, wherein the alkaline earth metal salt is a carbonate. 10. The ink-jet recording medium according to claim 7, wherein the undercoat layer contains an organic pigment 0.05 to 20 times the weight of the alkaline earth metal salt. 11. The inkjet recording medium according to claim 7, wherein the organic pigment is a hollow organic pigment or a dense organic pigment. 12. The inkjet recording medium according to claim 7, wherein the organic pigment is a mixture of a hollow organic pigment and a dense organic pigment in an amount of 0.1-10 times the weight of the hollow organic pigment. 13. The inkjet recording medium according to claim 11, wherein the hollow organic pigment has an average porosity of not less than 20%. 14. The inkjet recording medium of claim 11, wherein the dense organic pigment is bowl-shaped. 15. The inkjet recording medium according to claim 7, wherein the organic pigment has an average particle diameter of 0.3-10 micrometers. 16. The inkjet recording medium according to claim 7, wherein the inorganic ultrafine particles are amorphous synthetic silica or alumina compounds prepared by a gas phase method. 17. The inkjet recording medium according to claim 7, wherein the undercoat layer comprises 0.05-0.8 times the binder of the total solid weight of the alkaline earth metal salt and the organic pigment. 18. A method for producing an inkjet recording medium, comprising forming an undercoat layer on a carrier, forming an ink receiving layer thereon by coating a coating solution comprising inorganic ultrafine particles on the undercoat layer, the undercoat layer The layer comprises an alkaline earth metal salt and a binder whose content is 0.05-0.8 times the weight of the alkaline earth metal salt, the pH value of the coating solution for the ink receiving layer is not higher than 5.0, and after forming the undercoat layer, the undercoat The layer is subjected to heat calendering treatment, and then an ink-receiving layer containing inorganic ultrafine particles is formed. 19. The method for producing an inkjet recording medium according to claim 18, wherein said inorganic ultrafine particles are amorphous synthetic silica or alumina compounds prepared by a gas phase method. 20. A method for producing an inkjet recording medium, which comprises forming an undercoat layer on a support and forming an ink receiving layer thereon by coating a coating solution containing inorganic ultrafine particles on the undercoat layer, the undercoat The layer comprises an alkaline earth metal salt, an organic pigment, and a binder whose content is 0.05-0.8 times the total solid weight of the alkaline earth metal salt and the organic pigment, and the pH value of the coating solution for the ink receiving layer is not higher than 5.0, and the ink receiving layer is formed After the undercoat layer, the undercoat layer is subjected to heat calendering treatment, and then formed to provide an ink receiving layer containing inorganic ultrafine particles. 21. The method for preparing an inkjet recording medium according to claim 20, wherein said inorganic ultrafine particles are amorphous synthetic silica or alumina compounds prepared by a gas phase method.